By way of example, the central axial portion may consist of a duct of any cross-section; of a continuous wire impregnated with any filter and/or flavouring and/or coloring material; of filter material of different color and/or differently drugged, etc.; or of a series of cavities, each accommodating a flavouring capsule.
In the tobacco industry, it is known to implement a continuous filter rod using a forming machine provided with an inlet unit, the function of which is to unwind, from a bale of compressed filter material, a continuous tow consisting of a bundle of curled fibers of filter material; expand the tow until it takes the shape of a flat band by subjecting it to axial and cross stretching operations, through which the filter curling is staggered with respect to each other; and supply the band to an inlet station of the forming machine by means of a supply device adapted to transversally deform the band so as to transform it into a continuous cord generally having a cylindrical section. A possible “drugging” of the filter material by the addition of flavours, colorants or other additives normally occurs by suitably treating the band of filter material before the latter enters into the supply device and is deformed for forming the continuous filter rod. A common technique of “drugging” treatment of the band of filter material consists, for example, in depositing additive material in the form of a continuous layer or a discrete sequence of sediments on the upper surface of the band so that, when this is deformed by the supply device, the additive material remains incorporated into the filter rod.
According to a different technique, known from U.S. Pat. No. 3,910,166, before supplying the band to the supply device, the band “drugging” treatment includes cutting the band in longitudinal direction so as to form two strips, depositing a layer of additive material on one of the strips and overlapping the strips on each other so as to form again a band of filter material to be supplied to the supply device for forming the filter rod.
The resulting continuous rod is then cut into segments, normally having a multiple length of a normal filter, by means of a rotating cutting head.
If special filters of the above-described type are to be manufactured, it is known to provide, at the inlet station of the forming machine, a forming device of said central axial portion. This device may comprise a mandrel for making an axial duct of any section, or a tubular probe associated with a supply of a continuous wire or of a particulate material consisting, for example, of a flow of flavouring capsules or a flow of granules.
Moreover, if special filters of the above-described type are to be implemented, and for the precise purpose of perfectly centering said central axial portion with respect to said peripheral axial portion, it is known to equip the forming machine with two inlet units, which once the respective “tows” have been picked up from respective bales and have been subjected to the above-described processing series, supply the respective continuous cords so as to tighten said probe between each other as a “sandwich” and keep it perfectly centered.
A similar way of proceeding is not always effective and free from drawbacks.
In the first place, with the exception of the axially pierced filters, implementing the peripheral portion of a special filter capable of producing an acceptable pressure drop using two “tows” often is an expensive operation and not always possible since “tows”, which are characterized by the titer and number of fibers they are made of, are only available in a limited number and it is not always possible to find a “tow” on the market capable of forming a half peripheral portion of a “special” filter having the desired pressure drop values; moreover, such a “tow”, when available, normally is relatively expensive.
In the second place, since it is not possible to find two identical “tows”, since the two “tows”, even if identical by titer and number of fibers, cannot have an identical “warehouse history”, the two parts of the peripheral portion obtained by using these two “tows” have partly different physical-chemical features. Especially in the production of filters for “thin” cigarettes, such “differences” may be a different response of the two “tows” to treatments, with consequent offset of the central axial portion; and in any case they could cause, in a short time, the onset of internal tensions by effect of which the resulting filter segments tend to get deformed (they normally bend) and to become unusable for subsequent processing.